Special symmetries: carbon nanotubes
Transcript of Special symmetries: carbon nanotubes
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Special symmetries: carbon nanotubes
Yves Noël
and Raffaella Demichelis
id854261015 pdfMachine by Broadgun Software - a great PDF writer! - a great PDF creator! - http://www.pdfmachine.com http://www.broadgun.com
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Overview
I.What are carbon nanotubesa. Definitionsb. Vectors and the flat latticec. The helical symmetry
II.The exploitation of symmetry in CRYSTALa. Structure constructionb. Integrals calculationc. Diagonalisationd. Performance
III.Some results on carbon nanotubesa. Structureb. Band gapc. Vibrational frequencies
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What are nanotubes?Nanotubes are " regular cylindrical nanometric structures "
Experimentally discovered by Iijima in 1991
Inorganic nanotubes
Single wall /multiple wall nanotubes
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Periodicity and lattices
Tube:1D lattice
flat equivalent:
The graphene sheet~ 2D lattice
+ cyclicity
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The graphene sheet as reference
R,L define the nanotube cellcell basis.
Tube axis
Rolling vector R
Longitudinal vector L
Nanotube cell
R and L are
orthogonal
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The graphene sheet as reference
Vectors are expressed in the graphene basis.
R
L
Grapheneunit cell
R
R = R/N= (2,1)n1=6 n2=3
R = (n1,n2) = (6,3)
L = (l1,l2) = (-4,5)
In general n1 and n2are not coprime:
N is the greatest common divisor
R passes N=3 times through nodes
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The graphene sheet as reference
The chiral angle is measured with respect to the zigzag rolling and ranges from 0 to 30°.
(n,n) : arm
chair
=30°
(n,0) : zig zag =0°
(6,3) : Chiral
=19.1°
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The graphene sheet as reference
Tube axis
a1
a2R
L
R , H : a new basis more appropriated to the nanotube
R
H
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The graphene sheet as reference
The nanotube cell and R,H basis
The cell contains 42 nodes
R
HL=1/14
HR=3/14
H
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The graphene sheet as reference
Order :
R/R = N = 3
The R translation can be applied N times before reaching the starting node
R
1 2 3
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Correspondence betweenFlat 2D and cylindrical 1D lattice
Graphene Nanotube
Translation R/NTranslation R = R/3
Rotation 2/NRotation 2/3
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The graphene sheet referenceThe H translation can be applied 14 times before reaching the starting node
HL=1/14
HR=3/14
H
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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Translation R/pTranslation R = R/3
Translation HR = 3R/14
Translation L/KTranslation HL = L/14
General translationTranslation H
Correspondence between Flat 2D and cylindrical 1D lattice
Graphene Nanotube
Rotation 2/NRotation 2/3
Rotation 6/14
Translation L/KTranslation HL = L/14
RototranslationRototranslation (6/14 , L/14)
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The graphene sheet referenceThe H+R translation can be applied 42 times before reaching the starting node
HL=1/14
HR=3/14
HR
1
2
3
4
25
27
11
12
13
14
5
6
7
8
9
10
21
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23
24
26
30
31
15
16
17
18
19
20
29
32
33
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35
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41
4228
R+H
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Correspondence betweenFlat 2D and cylindrical 1D lattice
Graphene Nanotube
Translation R/NTranslation R = R/3
Translation HR = 3R/14
Translation L/KTranslation HL = L/14
General translationTranslation H
Rotation 2/NRotation 2/3
Rotation 6/14
Translation L/KTranslation HL = L/14
RototranslationRototranslation (6/14 , L/14)
Translation H+RRototranslation (3/14.2+1/3.2 , L/14)
Rototranslation (23/42.2 , L/14)
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Group symmetry
The nanotube symmetry group is the direct product of 2 cyclic groups:
An helical cyclic group of order M = N x K
A pure rotation of order N(R is larger than R )
An helical group of order K(coming from H)
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Large atom number but high symmetry
15.39
15.39
11.29
15.39
27.98
4.27
L(Å)
6.2213.92081042(12,4)
4.2513.9156782(9,3)
3.1119.184422(6,3)
2.8313.9104522(6,2)
2.577.6172862(6,1)
2.350.024122(6,0)
R(Å)
deg)
Nb atNbsym
Nb irr
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Exploitation of symmetry in CRYSTAL
I. For the tube constructionDISTINCTIVE NANOTUBE VECTORS EXPRESSED IN THE SLAB UNIT CELL BASIS A AND B
A BROLL UP DIRECTION, R 9 3HELICAL DIRECTION, H 2 1CYLINDER AXIS, L 1 5
LENGTHS IN ANGSTROMSCYLINDER RADIUS 3.112PERIOD ALONG THE CYLINDER AXIS, |L| 11.289THICKNESS OF THE SLAB 0.000
NUMBER OF ATOMS 84CHIRAL ANGLE 19.107 DEG
NANOTUBE SYMMETRYNUMBER OF SYMMETRY OPERATORS 42NUMBER OF ATOMS IN THE ASYMMETRIC UNIT 2THE HELICAL OPERATOR:ORDER 42ANGLE ( 23/ 42) x
2PI RADTRANSLATION 1/ 14 OF
THE CYLINDER PERIOD
DISTINCTIVE NANOTUBE VECTORS EXPRESSED IN THE SLAB UNIT CELL BASIS A AND B
A BROLL UP DIRECTION, R 9 3HELICAL DIRECTION, H 2 1CYLINDER AXIS, L 1 5
LENGTHS IN ANGSTROMSCYLINDER RADIUS 3.112PERIOD ALONG THE CYLINDER AXIS, |L| 11.289THICKNESS OF THE SLAB 0.000
NUMBER OF ATOMS 84CHIRAL ANGLE 19.107 DEG
NANOTUBE SYMMETRYNUMBER OF SYMMETRY OPERATORS 42NUMBER OF ATOMS IN THE ASYMMETRIC UNIT 2THE HELICAL OPERATOR:ORDER 42ANGLE ( 23/ 42) x
2PI RADTRANSLATION 1/ 14 OF
THE CYLINDER PERIOD
CRYSTAL computes the helical symmetry of the rolling vector given in the input and apply all the symmetry operators of the group to fill the tube.
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Exploitation of symmetry in CRYSTAL
II. For integrals calculation
CRYSTAL uses the symmetry to compute the mono and bielectronic integrals of the irreducible Fock matrix only
Irr F
+ Symmetry
F
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Exploitation of symmetry in CRYSTAL
III. For the diagonalisation in the reciprocal space of the Fock matrix
If a Symmetry Adapted Crystalline Orbitals (SACO) basis is used instead of a Bloch Functions basis, F(k) becomes block diagonal ; each block corresponding to an irreducible representation.
Example of the (6,0) SWCNT :
61111G :22 AOs24 atoms
F(k) size = 528x528
SACO12 blocks
44x44BF
1 block 528x528
Several small matrices are more easily diagonalised than a unique big matrix.
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Exploitation of symmetry in CRYSTAL
The unit cell of the (24,0) SWCNT contains 96 atoms.
FREQUENCY CALCULATION: 96x3+1=289 SCF calculations
With the helical symmetry exploitation, the calculation is performed on 2 irreducible atoms:
FREQUENCY CALCULATION: 2x3+1=7 SCF calculations
IV. building of the dynamic matrix (vibration frequency calculation)þ
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SCF cycle time scaling
Times of the different parts of an scf cycle(seconds as a function of the atoms number) þ
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Results
Computation conditions:Basis : 6 1111 G*Hybrid hamiltonian B3LYP
Studied carbon nanotubes : Zigzag nanotubes (n,0) up to n=24
References:1) Y. Noel, P. D�arco, R. Demichelis, C.M. Zicovich-Wilson, R. Dovesi On the use of symmetry in the ab initio quantum mechanical simulation of
Nanotubes and related materials, J. Comp. Chem, 2009, in press
2) Demichelis, D�arco, Noel, Dovesi � in preparation
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Results: Relaxation energy
E = Erelaxed - Eunrelaxed
Relaxation energy E (kJ/mol by cell), as a function of the radius (Å)
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Results : Stability
E = Erelaxed - Egraphene
Relative stability E (kJ/mol by cell) of the tube with respect to graphene, as a function of the radius (Å)
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Cyclicity and Brillouin zone
K
K
M
Graphene
b1
b2
K
SWCNT (n,0)
(6,0)
K
(7,0)
(9,0)
K
(8,0)
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Cyclicity and Brillouin zone
(4,4)
SWCNT (n,n)
K
K
M
Graphene
b1
b2
(3,3)K
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Cyclicity and Brillouin zone(6,0)
K
(3,3)
No moreon K Still on K
Semi conductor with small gap
conductor
But the Brillouin zone is deformed and is no more rigorously hexagonal because of the tube curvature
Metallic if 2n1+n2 or n1+2n2 is a multiple of 3
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Results : Band gaps
Energy gap as a function of the radius (Å)
Unpublished material removed
* Demichelis, D�arco, Noel, Dovesi � in preparation
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Results : Band gaps
(a) Weisman et al. Nano Lett. 2003, 3, 1235(b) Ouyang et al. Science 2001,292,702
Pure DFT methods systematically underestimate Eg; the inclusion of the exact HF exchange (hybrid methods) redresses this lack.
Hybrid functionals fail in describing small Eg; pure DFT provide more reasonable results.
Brothers et al. J. Phys. Chem. B
2006, 110, 12860
Unpublished material removed
* Demichelis, D�arco, Noel, Dovesi � in preparation
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Results : vibrational frequenciesFrequency calculation of (n,0) zig-zag SWCNT
645 (inactive) þ
0
1634
GRAPHENE
� 3N- 4 vibrational frequencies (N=atoms in unit cell; N=4n for (n,0) SWCNT) þ
� 15 active modes for all (n,0) SWCNT
� Frequencies tend to the ones of graphene for increasing SWCNT radii
6 modes
3 modes
6 modes
Unpublished material removed* Demichelis, D�arco, Noel, Dovesi � in preparation
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Practical: a nanotube input
Specific keywords SWCNT
construction
NANOTUBE
construction
NANORE
Use a previous geometry
NANOJMOL
Graphical output
Usefull keywords ROTCRY
Position of the atomswith respect to theundistorded surface
ATOMROT
Which side in/out